arm64: mm: Add arm64_kernel_unmapped_at_el0 helper

arm64: mm: Add arm64_kernel_unmapped_at_el0 helper

CVE-2017-5754 ARM64 KPTI fixes

In order for code such as TLB invalidation to operate efficiently when
the decision to map the kernel at EL0 is determined at runtime, this
patch introduces a helper function, arm64_kernel_unmapped_at_el0, to
determine whether or not the kernel is mapped whilst running in userspace.

Currently, this just reports the value of CONFIG_UNMAP_KERNEL_AT_EL0,
but will later be hooked up to a fake CPU capability using a static key.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Tested-by: Shanker Donthineni <shankerd@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
(cherry picked from commit fc0e1299da548b32440051f58f08e0c1eb7edd0b)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>